The present invention relates to a radio handset for detecting its current position using a radio signal and in particular, to a radio handset and a position location system for accurately identifying its current position even when it is difficult to discriminate a base station from which the signal has been transmitted.
In a mobile communication system, there has been suggested a technique to detect a position of a radio handset by using a signal transmitted from a base station. For example, JP-A-7-181242 (laid open on Jul. 21, 1995) discloses a technique for detecting a handset position by using positions of base stations and propagation delay time differences between signals transmitted to a handset from different base stations in a code division multiple access (CDMA) system.
For example, in a radio communication system shown in FIG. 11, a radio handset 800 receives signals from three cellular phone base stations 801, 802, and 803. The radio handset 800 calculates signal propagation time differences, i.e., [propagation time of the signal transmitted from the base station 801xe2x80x94propagation time of the signal transmitted from the base station 802] and [propagation time of the signal transmitted from the base station 801xe2x80x94propagation time of the signal transmitted from the base station 803]. Using the propagation time differences, the radio handset 800 calculates a current position of the radio handset 800.
Here, explanation will be given on a system based on the EIA/TIA-95 specified by the TIA (Telecommunication Industry Association) which is an US CDMA digital moving communication system. In the EIA/TIA-95 system, all the base stations use the same spread code (PN code). Moreover, each of the base stations has a pilot PN offset (PN offset value) as a transmission time difference predetermined for each of the base stations for identification thereof and transmits a signal at a timing shifted from the standard timing by its PN offset value.
A radio handset calculates correlation between the received signal and the PN code so as to obtain a delay profile. For example, a correlation value between the received signal and the PN code is calculated by using a matched filter, the matched filter outputs a delay profile as shown in FIG. 12.
In FIG. 12, 901. 902, and 903 shows correlation values corresponding to signals received from the base stations 801, 802, and 803, respectively. Here, since the base stations have different PN offset values, the correlation values corresponding to the signals from the base stations are temporally separated from one another. Accordingly, in the delay file, it is possible to isolate each of the signals transmitted from the different base stations as well as received timings of the signals from the base stations. Thus, it is possible to detect a location by using the signal propagation time from the base stations.
However, in a large city where base stations are concentrated, there is a case that base stations having the same PN offset value are located in the proximity to each other. For example, when one and the same PN offset is assigned to the base station 801 and the base station 802, a delay profile waveform as shown in FIG. 13 is obtained as matched filter outputs corresponding to the base stations 801, 802, and 803. That is, since the base stations 801 and 802 have the same PN offset value, correlation values corresponding to the signals received from these base stations are overlapped at the same time as a delay profile waveform (matched filter output).
Thus, when the received signals appear almost at the same time in the delay profile, it is difficult to isolate, according to the base station, a received timing of one signal from another signal. That is, there is a high possibility that this correlation value 910 is incorrectly correlated to a signal from a particular base station. Such a mistake introduces a large error to the propagation time measurement of signals from the respective base station, thereby deteriorating the location accuracy. This means that a user cannot receive an appropriate location information service. That is, benefit of the location information service is diminished.
It is therefore an object of the present invention to provide a radio handset capable of accurately measuring its current position even when it is difficult to isolate respective signals from different base stations.
The radio handset according to an aspect of the present invention includes: signal reception means for receiving signals from a plurality of radio stations; reception timing analysis means for analyzing reception timings of signals received by the signal reception means; radio station selection means for selecting radio stations to be used in a position calculation; and position calculation means for calculating a position of a signal reception point by using the reception timings of the signals from the selected radio stations; wherein the radio station selection means selects the radio stations to be used in the position calculation in such a manner that when reception timings of signals from two or more of the radio stations cannot be separated from one another (for example, when a plurality of radio stations have an identical PN offset value and a delay profile base don the signals from two or more radio stations are overlapped), it is decided to exclude a signal from at least one of these radio stations.
According to another aspect of the present invention, the radio handset includes reception means for receiving signals from a plurality of radio stations, delay profile creation means for creating a delay profile of the signals received by the reception means, and information transmission means for transmitting information used for calculating a position of a reception point using the delay profile, wherein when signals from two or more radio stations cannot be separated from one another in the delay profile (for example, when a plurality of radio stations have an identical PN offset value), the information transmission means transmits information used to exclude at least one of the signals from these radio stations.
According to still another aspect of the present invention, there is provided a position location system including a plurality of radio stations for transmitting signals to a radio handset, reception means for receiving signals from the plurality of radio stations, and a radio handset having delay profile creation means for analyzing a delay profile of signals received by the reception means, wherein position calculation means is built in the radio handset or is provided separately for calculating a position of a reception point by using the delay profile, and wherein the position calculation means calculates the position in such a manner that when signals from two or more of the radio stations cannot be separated from one another in the delay profile (for example, when the plurality of radio stations have the same PN offset value), at least one signal thereof is excluded from the position calculation.
According to yet another aspect of the present invention, there is provided a position calculation apparatus including radio station selection means for selecting radio stations to be used in a position calculation, and position calculation means for calculating a position of a reception point by using reception timings of signals received by a radio handset from a plurality of radio stations, wherein in case reception timings of signals from two or more of the radio stations cannot be separated from one another (for example, when a plurality of radio stations have an identical PN offset value and delay profiles based on the signals from these radio stations are overlapped), the radio station selection means selects radio stations to be used in a position calculation in such a manner that a signal from at least one of these radio station is excluded.
In the radio handset according to the aforementioned aspects of the present invention, in case reception timings of signals from two or more radio stations cannot be separated from one another, radio station selection means excludes a signal from at least one of these radio stations when selecting radio stations to be used in the position calculation. Accordingly, even in an area where radio stations having an identical PN offset value are located in the proximity from each other, it is possible to calculate a current position of the handset with a high accuracy.